To ensure proper toner development and print quality in liquid toner printers and copiers, the liquid toner concentration must be measured and controlled in the liquid toner electrophotographic print process. Several techniques to measure liquid toner concentration or percent solids by weight include optical density, conductivity, permittivity, density and ultrasonic methods. Because high concentration liquid toners have a high light extinction factor, optical techniques are limited to measurement below a few percent solids. Where the operating point is higher, typically approximately eight percent solids, an optical technique is unacceptable. The liquid toner conductivity, permittivity, and density signal to noise ratio is small for changes in toner concentration. High frequency ultrasonic techniques overcome these problems, but are expensive to implement.
U.S. Pat. No. 5,208,637 to Landa discloses optical sensing for replenishment of liquid toner. U.S. Pat. No. 4,343,548 to Bares et al discloses sensing conductivity to replenish toner. This invention uses a transverse flow viscometer, which is a known viscosity measurement technique, to predict the liquid toner concentration. This invention provides a low cost sensor with improved percent solids range and signal to noise ratio.
The liquid toner viscosity is a function of fluid temperature, percent solids, and toner particle size distribution. For a constant particle size distribution, the liquid toner concentration may be predicted by measuring the fluid temperature and viscosity. This invention applies a transverse flow viscometer technique to predict the liquid toner concentration. Transverse flow viscometers and basic theoretical equations employed to develop this invention, are described in the book Viscosity and Flow Measurement, A Laboratory Handbook of Rheology by J. R. Van Wazer et al. U.S. Pat. No. 3,368,390 to Norcross is illustrative of known activity to flow viscometers.